Electron stream transmission device



NOV. 18, 1969 McGEE ET AL ELECTRON STREAM TRANSMISSION DEVICE Filed NOV.26, 1965 SD k\\\\\\\\\\\\\\\\\\\\\\\\\\\ INPUTS s m e m m y m 0 13/3 860.v 6 w W5 HE D p 2 M 66 am FR M MJflm/M m M MW 1-- m q -1- n u a n n u uM m m m; N0 mm GF United States Patent 3,479,516 ELECTRON STREAMTRANSMISSION DEVICE James Dwyer McGee, London, John Beesley, HighBarnet, and Anthony David Berg, London, England, assignors to NationalResearch Development Corporation, London, England, a British corporationFiled Nov. 26, 1965, Ser. No. 509,805 Claims priority, application GreatBritain, Nov. 27, 1964,

Int. Cl. H01j 39/12 US. Cl. 250-213 9 Claims ABSTRACT OF THE DISCLOSUREA vacuum tube device suitable for timing, informationstorage,information-translation and for detection and display of high-speedtransients essentially comprising a vacuum envelope, an input electrodefor responding to one or a multiplicity of signals and for transmittingone or a parallel multiplicity of electron streams in response thereto,a spaced pair of mesh screens spaced to provide an electron stream pathof nanoor micro-second transmission length, a tubular electrodesurrounding the electron stream path between the mesh screens, magneticfocussing means surrounding the vacuum space, and control circuits forcontrolling the mesh screens to act alternatively as electron gates andreflectors so that electron streams can be allowed to enter and leavethe electron path between the screens at will and to reciprocate betweenthe screens for up to millisecond periods for storage purposes, it beingpossible to selectively extract electron streams from a storedmultiplicity.

This invention relates to vacuum tubes and devices incorporating suchtubes, and has for its object tubes and devices suitable for timing,information-storage, information-translation, and like purposes, and inconnection with the detection and display of high-speed transients.

The main aspect of the invention comprises a vacuum tube within whichare mounted axially in line in spaced relation, means for generating anelectron stream of varying intensity, a first screen, a second screenspaced from the first screen by a distance requiring the use ofelectronfocussing means to ensure electron transmission between pointshaving the same coordinates on the two screens, and electron-responsivemeans beyond the second screen, and which comprises electrode meansencircling the space between the screens, first accelerator/retardermeans immediately following said first screen, and secondaccelerator/retarder means immediately preceding said second screen,said screens, electrode means, and accelerator/retarder means being somounted as to be individually controlled in potential.

Variation in the electrical condition of the screens will either allowfree passage of electrons therethrough, or alternatively reflectelectrons impinging thereon. The screens, acting as reflector gates, canwholly or partly span the tube, and a plurality of partial screens canbe arranged to be complementary in that respect, to be at differentlocations in the tube, and to 'be separately controllable. Such tubescan form part of equipment by means of which the electron-streamgenerating means is activated by pulses: for example, light pulses; toset up pulse emission; and the screens potentials are varied to controlthe passage of pulse trains between and through the screens.

axis, so forming a number of transmission channels along the tube.

Electron pulse trains forming such channels can reciprocate to and fro,between the screens currently acting as reflectors, without losing theiridentity. By activating a cathode spot by a succcession of light pulses,a sequence of discrete bunches of electrons is emitted along thecorresponding channel: for example, a sequence of light pulsescorresponding to the 1s of a binary information train will generate aninformation train of electron bunches along the channel. After passagethrough a first reflector-gate biassed to act as a gate, such anelectron train can reciprocate or circulate along the channel betweensaid first reflector gate and a second reflector-gate while both arebiassed to the reflecting condition.

Reciprocating electron pulses can maintain direction and identity for alarge number of passes along the tube, so that while the time taken fora single electron pulse to traverse along the tube is of the order ofnanoseconds and a single reciprocation takes microseconds, electronpulse trains can reciprocate for periods of the order of millisecondsand still be recognisable.

The tube alone also has utility for nanosecond, microsecond andmillisecond timing, time position transfers, and code translationpurposes.

However, as in the case of solid-column and liquidcolumnpulse-circulation stores, it is possible to regenerate electrontransmission by opening the outgoing reflector-gate at a suitablemoment, detecting the electron bunches, amplifying and/or reshapingresulting electrical signal trains, and utilising them to generate acorresponding train of light pulses which are applied to thephotocathode to start a regenerated electron train similar in content tothe original train. This process can be continually repeated so thatstorage can be prolonged for an indefinite period.

It is apparent that information-a word, for examplecan be introducedinto the tube either in series in a single channel, or in parallel in anumber of channels, and a number of channels can be used to reciprocateor store a sequence of parallel words, or any other arrangement ofinformation, on a three-dimensional basis.

Further, since an electron stream in a channel can reciprocate withoutinterference between oppositely moving electrons, a channel can store astream or train of electrons longer than the distance between a pair ofclosed gates, so that any length of train up to two channel lengths canbe reciprocated and extracted without confusion.

The maximum number of binary bits which can be stored in such a tube isequal to the number of bits which can be stored in a single channel withminimum spacing of the bits multiplied by the number of channels.

Several trains of pulses having pulse spacings which are multiples ofthe minimum bit-storage of which a channel is capable, can be stored ina single channel on an interpolation basis, so that each pulse positionin a repetitive cycle of x pulse positions is allocated to a differentone of x pulse trains.

While the invention has wide possibilities of use, it would appearcapable of fulfilling an important need in very high speed computers:namely, the provision of a very high-speed store.

The invention will be described with reference to the accompanyingdrawings in which:

FIG. 1 illustrates schematically the basic construction of a storagedevice embodying the invention, while FIGS. 2 and 3 illustrate exemplaryforms of selectivelyoperable reflector-gates for use in the storagedevice of FIG. 1.

Since their conception, digital calculators have been limited in theirperformance by the size and speed of their information storagefacilities. A significant step forward in speed and capacity came withthe introduction of the acoustic relay line. The electro-static storagesystem of F. C. Williams superseded this and finally the ferrite corestore has become the basic component of all the fast computer stores.

For even faster machines the finite speed of propagation ofelectromagnetic signals becomes important and feeding of informationinto and out of circuit elements (fan in and fan out) is a furtherproblem because of reflection of step wavefront's-necessitating the useof matched lines.

An opto-electronic system (as proposed by Cooke-Yarborough: Proc I.E.E.,vol. 111, No. 10, page 1641, 1964) has many advantages. Coupling betweenarithmetic units can be accomplished optically (e.g. using light fibrepipes), considerably reducing cross-talk and increasing the usublefan-in and fan-out ratios.

Computer systems utilising piped light and photo-electronic high gainrepeater tubes operating at electron speeds are capable of very fastcomputation speeds, and nano-second working can be achieved. To make useof such a system, it is necessary to have also a high-speed,high-capacity store and this is the object of the present invention.

FIG. 1 shows a photo-electronic vacuum tube device capable of acting asa computer store at nano-second and micro-second speeds. One end of thedevice is a photo-emissisive cathode PEC while the other end is asemi-conductor detector device SCD. The broken vertical lines RG 1, 2within the tube indicate mesh screens capable of acting as gates and asreflectors for electrons passing along the tube in response to differingelectric potential conditions applied thereto via switches SW1, SW2. Thegroup of full vertical lines at each end of the region between thescreens RG1, RG2 indicate groups of .annuli AN which can act as electronaccelerator/retarders (according to direction of electron travel) inresponse to progressively-varying potentials applied to the annuli ofeach group, preferably by a potentiometer control from the respectivescreen, as indicated in FIG. 1. Of course, there will be an electrode,for instance a cylindrical metal liner to the tube, in the region ER ata suitable positive potential.

A solenoid SD surrounds the tube and its magnetic field controls boththe formation and the diameters of the helical paths followed by theelectrons in each channel along the equipotential region ER in the tube.

The photo-emissive surface of the cathode PEC will emit electrons whenilluminated with photons in 10- sec., and provides fast Read-in. Usinglight pipes to feed the information to elemental cathode areas,cross-talk is considerably reduced and the fan in ratio; that is, thenumber of discrete information-responsive areas; can be high.

The photo-electrons themselves are used to store the information.

The screen or mesh RG succeeding the photo-cathode controls thephoto-electron stream. By biasing the mesh to a suitably negativepotential the photo-electrons are all returned to the photocathode.

If now the mesh is turned positive, the electron stream passes throughand, after acceleration by the left-hand annuli, enters theequipotential region, through which it drifts at a speed controlled bythe voltage applied to the equipotential section of the tube. The pathfollowed by the electrons is a very tight helix, the diameter of whichis governed by the magnetic field produced by the solenoid SD, in whichthe whole tube is immersed. Electrons with 100 ev. energy travel at aspeed of about 5.9 m./ sec. Thus if the equipotential region is about 60cm. in length, the transit time of such electrons will be about 100nanoseconds. The spatial resolution is maintained by the large magneticfield. Electron streams passing along the equipotential region areaccelerated or are retarded as desired by the right hand annuli AN beingencountering the second mesh RG2 by which the electron flow can becontrolled. If mesh RG2 is positive the electrons can pass through. Ifthe mesh is negative they are reflected and return down the tube. Thusafter a time equal to twice the one-way transit time an electron willagain reach the first mesh RG1. If mesh RG1 has been turned negativebefore the electrons reach it on their return journey, they will againbe reflected. The electron information fed in over a period of 200 nsec.is trapped between the screens and forced to oscillate back and forth.

Each fully resolvable discrete area or picture point on the photocathodeis the source of a stream of electrons which will be nearly modulated bya random stream of photon pulses incident on the picture point. Thismodulation can be maintained through many traverses of the electronstream up and down the tube. If such an electron stream can be modulatedat nanosecond rate and the spatial resolution of the image is 100picture points in each direction (giving a coordinate array of 10,000points), then the total number of bits of information trapped in thetube during 200 nsec. will be 200 (100) =2.l0 comprising 200 bits ineach of 10,000 storage channels.

It can be shown that, by suitably choosing the length of theequipotential drift and the speed-control region at each end, anydilference in initial axial energies will introduce only smalldispersion in their time of arrival. This can be seen qualitatively byconsidering first the equipotential region. An electron with largeemission energy will travel faster and thus take less time to completeits journey than an electron leaving the photocathode with low energy.When the decelerating region is reached, however, the extra energy nowhas the opposite effect. The more energetic electron penetrates furtherinto the retarding field and thus spends longer in this region. Electronkinetic considerations lead to a very simple relation which balancesthese two effects and yields zero time dispersion for any value ofelectron emission energy, V, where V is the voltage on the equipotentialregion. This information fed into the tube can be stored over manytransits with virtually no loss of time resolution.

Two main features govern the maximum storage time possible.

(1) residual gas pressure,

(2) non-linear regions in the electric and magnetic fields.

Due to the direct relation between the storage time and electron pathlength (for any particular voltage on the equipotential region) the meanfree path and hence the residual gas pressure in the device control themaximum storage time. With reasonably good vacuum techniques, electronsare not likely to make a gas collision more than once in ten thousanddouble transits, or an electron pulse (information bit) could make athousand double transits before it would have lost 10% of its electrons,which is about the attenuation allowable before the pulse becomesunrealiable. Any non-linearity in the electric and magnetic fields addgeometrical distortions and loss of axial energy on each transit andhence limit the maximum number of transits possible before the loss ofimage resolution becomes intolerable. These fields can be made almost asuniform as desired and hence it becomes feasible to make this electronimage information store such that there is no serious loss ofinformation in, say, 10 microseconds.

The second mesh can be switched on and off in l or 2 nseconds and thiswill let through that portion of the electron streams arriving at themesh during that period. Detection and amplification for the electronoutput of each channel may be provided by the semi-conductor terminationSCD comprising, for example, suitablybiassed semiconductor p-n junctionswhich will produce an electron hole pair for each 3 ev. incident energywhen struck by an energetic electron; or an electron multiplier array.In this way, the electron pulses are accelerated up to, say, 20 kv., sothat a gain of about 6,700 is achieved.

Because of the accelerating fields, the transit time of the electron andhence the delay involved in the amplification is short (1 nsec.) In thisway an output current is generated if an electron passes through meshRG2 when it is acting as a gate. Output current from a channel isapplied to a corresponding light diode (not shown) and the system iscomplete.

Computer techniques involve the use of serial or parallel machines. In aserial machine the bits which go to make up a word are handled insequence. In a parallel machine all bits of one word are processed atthe same time. If a 50 bits/ word system is under consideration, clearlya speed gain of 50 is potentially available by operating in the parallelmode.

The store described is adaptable for use in either mode; the variousproblems associated with each will be con sidered individually.

Serial operation A serial binary 50-bit word can be read into the storeby applying a corresponding pulse series to a light diode which iscoupled to a resolution element of the input photocathode. The word isthus stored in the channel associated with the resolution element as aseries of electron bunches corresponding to the 0s and 1s of the binarysystem. If the fundamental time element is 1 nsec., the 50 bit Word willoccupy 50 nsecs. The individual wires of mesh RG2 are now electricallyinsulated as indicated in FIG. 2. For example, the horizontal andvertical sets of wires of mesh RG2 can be slightly spaced axially. Ifall the wires are biased negative, FIG. 2, the storage or reflectorstate is defined, and the electron information will oscillate back andforth. If, however, adjacent pairs of horizontal and vertical wires arepulsed positive, as shown, one region of the mesh bounded by the fourwires and constituting an electron channel will let a stream ofinformation through. Thus a word can be removed from the store.

FIG. 3 shows an alternative arrangement of mesh comprising twoaxially-spaced coordinate wire grids RGZA, RGZB staggered in bothdirections in relation to one another.

Grid RGZA ha a fixed potential while the gating grid RGZB is formed likethe mesh of FIG. 2. In this case however a single wire in each directionis given positive potential in order to open the fixed grid channel CHwhich contains the cross-point of the two positive wires.

Parallel operation For operation in the parallel mode all 50 bits arefed in at the same time on to the front photocathode and enter the 50channels in the store simultaneously. Thus a particular word nowoccupies a slice of the equipotential region normal to the axis and canbe removed by pulsing on the second mesh RG2 to allow all the bits ofthe word to pass through simultaneously. By arranging a suitable numberof output channels the/whole word can be processed at once.

When information is read out of this store, the stored information iseliminated. However, the information, besides going to the othersections of the computer can also be fed back into the store by applyingcorresponding signals to the input; if necessary after a suitablepredetermined time delay. All stored information can 'be extractedperiodically, reshaped, amplified, and reinjected into the cathode endof the tube. One or more channels may be fed with a series of codedsignals to act as identifying or indexing signals for the storedinformation.

In FIG. 2, by applying positive potential to all the vertical Wires, forexample, and also to two adjacent horizontal wires only, negativepotential being applied to the remaining horizontal wires, a line oftransmission apertures can be opened to pass a line of bitsconstituting, for example, a complete word.

When it is desired to store on a line basis only, the outgoing screencan be a set of parallel horizontal, or vertical, wires only, and a lineof bits will be read by applying positive potential to one wire only,while the other wires are biased negative. In all the above cases, theincoming screen can be of the same construction as the outgoing screen,or can be of a different construction: for example, a wholly ON-OFFincoming screen can be associated with an outgoing screen of one of themore sophisticated constructions discussed above.

What we claim is: 1. An electron stream transmission device comprisingin a vacuum:

means for generating an electron stream of variable intensity,

an output device directly responsive to electrons from the stream andseparated from the generating means by a straight electron transmissionpath having a length such that for a direct transit of the stream alongsaid path there is negligible transmission loss due to electronsstriking gas molecules,

a tubular electrode disposed between the generating means and the outputdevice so as to be coaxial with said transmission path,

a first mesh screen which is disposed across said path adjacent saidgenerating means,

a second mesh screen which is disposed across said path between thetubular electrode and the output device, and comprising means forproviding a focussing magnetic field extending along said transmissionpath.

2. An electron stream transmission device as in claim 1 wherein saidelectron-stream generating means is capable of generating simultaneouslya number of discrete parallel electron streams and said screens arecapable of passing and reflecting such discrete parallel electronstreams, and wherein said device comprises a like number of individualpulse inputs to said generating means, and a like number of individualpulse outputs from said electron-responsive means.

3. An electron stream transmission device as in claim 1 wherein saidelectron stream generating means is a photo-electric cathode.

4. Apparatus comprising an electron stream transmission device as inclaim 1, and comprising:

means for applying a pulse train to said generating means while thefirst screen is controlled to function as a gate allowing passage of anelectron stream from said generating means and said second screen iscontrolled to function as a reflector;

means for changing the cotnrol on the first screen after a time intervalfrom the commencement of said pulse train no longer than the timerequired for an electron to travel from the first screen to the secondscreen and back, so that the first screen now acts as a reflector sothat the electron stream shuttles back and forth between the screens;and

means for changing the control on the second screen after an arbitraryinterval so that the second screen now acts as a gate to allowtransmission of the electron stream which has been dynamically storedbetween the screens, to the electron-responsive means.

5. An electron stream transmission device as in claim 1 wherein at leastthe outgoing screen is comprised solely of a set of parallel wiresmounted so that individual electrical conditions can be applied to eachindividual wire.

6. An electron stream transmission device as in claim 1 wherein at leastthe outgoing screen is a coordinate wire mesh mounted so that individualelectrical conditions can be applied to each wire of the mesh.

7. Apparatus comprising an electron stream transmission device as inclaim 6, comprising:

means for biassing all the wires of a screen positive so that the wholearea of the screen will transmit electron streams;

means for biassing all the Wires of a screen negative so that the wholearea of the screen will reflect electron streams; and means for biassingadjacent pairs of vertical and horizontal Wires of a screen positivewhile all the other wires are biassed negative so that one region of themesh bounded by the four wires biassed positive will constitute anelectron stream transmission path while the remainder of the screen actsas an electronstream reflector. 8. An electron stream transmissiondevice as in claim 1 wherein at least the outgoing screen comprises ,tWoaxially-spaced coordinate wire grids staggered in both directions inrelation to one another, one of said grids being mounted so that acommon electrical condition can be applied to all the wires, and theother grid being mounted so that individual electrical conditions can beapplied to the individual Wires of the grid.

9. Apparatus comprising an electron stream transmission device as inclaim 8, comprising:

means for biassing said one grid positive so that the whole screen willtransmit electron streams; means for biassing said one grid negative sothat the whole screen will reflect electron streams; and

means for biassing said one grid negative and of biassing one verticalwire and one horizontal wire of said other grid positive, so thatfour-sided meshof the one grid which contains the cross-point of thepositively-biassed wires of the other grid con- 5 stitutes anelectron-stream transmission channel while the remainder of the area ofthe screen acts as an electron-stream reflector.

10 References Cited UNITED STATES PATENTS 2,640,162 5/1953 Espenschiedet al. 250213 X 2,919,377 12/1959 Hanlet 250213 X 3,310,678 3/1967Kylander et al. 250-213 X 15 3,335,284 8/1967 Parks 2502l3 X RALPH G.NILSON, Primary Examiner T. N. GRIGSBY, Assistant Examiner US. Cl. X.R.

